1. Field of the Invention
This invention relates to well borehole logging devices of the type designed to determine certain characteristics of the borehole for petroleum exploration and production, including the velocity of the fluid movement in the borehole.
2. Description of the Prior Art
Prior well logging devices and methods are known which feature a signal emitter and one or more signal detectors. U.S. Pat. No. 4,005,290, entitled "Neutron-Neutron Logging", issued Jan. 25, 1977, shows a borehole logging tool which utilizes a radioactive emitter and a plurality of neutron detectors. U.S. Pat. No. 3,056,463, entitled "Sonic Borehole Logging Devices", issued Oct. 2, 1962, shows an acoustic logging system which has a sonic emitter and acoustic transducers for detecting the emitted signal.
The velocity of fluid movement in the borehole is one characteristic of interest in well logging. By sampling the fluid velocity at various intervals along the borehole, the operator can obtain an indication of whether different perforated zones are producing, or whether fluid is being lost to a porous zone in the well bore. In the past, the above types of well logging devices were used in basically two ways to obtain an indication of the well fluid velocity. One method involved lowering a downhole device to a desired location within the well bore on a supporting cable. A signal was then produced from an emitter at one end of the downhole device and the signal was tracked by two or more detectors as the signal and associated well fluid moved past the detectors. The downhole device was held stationary by the supporting cable. Since the cross sectional area of the borehole was known and the distance between the detectors was known, the time taken for the signal to pass the detectors gave an indication of the fluid velocity or flow rate.
In the other method, the downhole device would typically be provided with a signal emitter and a single detector. The device would be lowered to a desired location and the signal emitted. The device would then be run ahead of the signal for a given distance, i.e., 50 feet, and the signal would be detected as it passed the device. Once again, since the distance of travel was known, and the time taken, the velocity or flow rate could be approximated.
While the above methods have been extensively utilized to obtain information regarding the characteristics of producing formations, formations used to store products, and formations used to dispose of waste fluids, several problems exist with their use. Certain of the prior methods required precise calibration of the instruments utilized. Additional difficulties resulted from the sensitivity of the methods employed to variations in temperature and pressure within the well bore. Where a stationary downhole device was utilized, only a discrete interval of the well bore could be sampled.